SCHWARZE NICKELOBERFLAECHEN UND VERFAHREN ZU IHRER HERSTELLUNG

The protective outer layer is formed by electrodeposition from a soln. contg. a dichromate or chromate, esp. sodium dichromate or potassium dichromate. The pref. aq. soln. used for electrodeposition of the protective layer contains 50-300 g/l of a Cr cpd. and is used for 0.5 mins. to a few mins. with current density 0.1 -2 A/dm2; and the soln. pref. also contains another salt increasing its conductivity. The protective layer is used esp. on the black nickel surface of a solar energy collector. Black Ni coatings have high absorption and low emission, but they corrode in damp atmos. and this defect has limited their use. The invention permits the black Ni to withstand e.g. a salt spray test of 72 hrs., or 5000 hrs. exposure to an atmos. with 98% humidity at 65 degrees C.